Drive and control circuits for gate capacitance latch with refresh lamp ballast

Abstract

A switched AC ballast is operated synchronously with the AC line during normal running of a gas discharge lamp to limit EMI and is locked in a single phase during start-up (i.e. ignition, glow mode and warm-up) to protect the bridge switches from high voltage starting signals. Devices having a high impedance control input, such as FETs, are employed in the switching bridge thus allowing control of the upper devices in the bridge without a floating power supply. A control circuit insures proper operation of the bridge switches during all operating modes of the discharge lamp.

What is claimed is: 
     
       1. A controller circuit for controlling an AC to AC converter in a ballast for a gas discharge lighting system, said converter including a diode rectifier for connecting to an AC source and providing a DC output voltage, a current-limiting impedance having one side connected to one side of said rectifier, and a full-bridge switching circuit coupled between the other side of said impedance and the other side of said rectifier for converting said DC voltage to an AC voltage, said full-bridge switching circuit including a pair of converter legs, each of said legs being comprised of a series connected pair of upper and lower semiconductor switching devices, each of said upper devices having a high impedance control electrode, said lighting system including an oscillator having its output terminal coupled to the output terminals of said full-bridge switching circuit for providing electrical energy to start the lamp of said lighting system, said controller circuit comprising: a diode conduction sensing circuit coupled to the inputs and the outputs of said rectifier for determining when the diodes in said rectifier are non-conducting;   an oscillator sensing circuit coupled to said oscillator for signaling a starting condition when said oscillator is operating;   an arc sensing circuit coupled to said converter and said oscillator for disablilng said oscillator when current in said lamp exceeds a predetermined value;   a pair of gate capacitance latch driver circuits, each connected to a respective upper switching device and each including a gate capacitor, charging means for charging said gate capacitor and discharging means for discharging said gate capacitor, each of said gate capacitors connected between the control electrode and a second electrode of its respective upper switching device; and   logic means for generating control signals to operate said lamp in starting and normal running conditions, said logic means being coupled to said diode conduction sensing circuit, said oscillator sensing circuit, said driver circuits and said lower switching devices, said logic means locking said full-bridge switching circuit in a single phase during a starting condition while simultaneously refreshing the upper switching device of the phase which is conducting each time said rectifier diodes become non-conducting during said starting condition, said logic means reversing the phase of said full-bridge switching circuit each time said rectifier diodes become non-conducting during a normal running condition.   
     
     
       2. The controller circuit of claim 1 wherein said logic means is coupled to each of said gate capacitors and generates a first control signal for charging a first one of said gate capacitors, a second control signal for charging a second one of said gate capacitors, a third control signal for turning on and off one of said lower switching devices, a fourth control signal for turning on and off the other lower switching device, a fifth control signal for discharging said first gate capacitor and a sixth control signal for discharging said second gate capacitor. 
     
     
       3. The controller circuit of claim 2 wherein each of said charging means is comprised of a diode having its cathode connected to the control electrode of its respective upper switching device and its anode coupled to said logic means, and wherein each of said discharging means is comprised of a discharge switch having its outputs coupled across a respective capacitor and a diode having its cathode connected to the control electrode of the respective discharge switch and its anode coupled to said logic means. 
     
     
       4. The controller circuit of claim 3 further comprising: an over-temperature sensing circuit coupled to said logic means and said arc sensing circuit for turning off said lower switching devices and disabling said oscillator when the temperature in said ballast exceeds a predetermined temperature.   
     
     
       5. A gas discharge lighting system comprising: a diode rectifier for connecitng to an AC source and for providing a DC output voltage;   a current-limiting impedance having one side connected to one side of said rectifier;   a full-bridge switching circuit connected between the other side of said impedance and the other side of said rectifier for converting said DC voltage to an AC voltage, said full-bridge switching circuit including a pair of converter legs, each of said legs being comprised of a series connected pair of upper and lower semiconductor switching devices, each of said upper devices having a high impedance control electrode;   a gas discharge lamp coupled between the outputs of said full-bridge switching circuit;   an oscillator having its output terminals coupled to the output terminals of said full-bridge switching circuit for providing electrical energy to start said lamp;   a diode conduction sensing circuit coupled to the inputs and the outputs of said rectifier for determining when the diodes in said rectifier are non-conducting;   as oscillator sensing circuit coupled to said oscillator for signaling a starting condition when said oscillator is operating;   an arc sensing circuit coupled to said full-bridge switching circuit and to said oscillator for disabling said oscillator when current in said lamp exceeds a predetermined value;   a pair of gate capacitors latch driver circuits, each connected to a respective upper switching device and each including a gate capacitor, charging means for charging said gate capacitor and discharging for discharging said gate capacitor, each of said gate capacitors connected between the control electrode and a second electrode of its respective upper switching device; and   logic means for generating control signals to operate said lamp in starting and normal running conditions, said logic means being coupled to said diode conduction sensing circuit, said oscillator sensing circuit, said driver circuits and said lower switching devices, said logic means locking said full-bridge switching circuit in a single phase during said starting condition while simultaneously refreshing the upper switching device of the phase which is conducting each time said rectifier diodes become non-conducting during said starting condition, said logic means reversing the phase of said full-bridge switching circuit each time said rectifier diodes become non-conducting during a normal running condition.   
     
     
       6. The lighting system of claim 5 wherein each of said charging means is comprised of a diode having its cathode connected to the control electrode of its respective upper switching device and its anode coupled to said logic means, wherein each of said discharging means is comprised of a discharge switch having its outputs coupled across a respective gate capacitor and a diode having its cathode connected to the control input of the respective discharge switch and its anode coupled to said logic means, and wherein said logic means generates a first control signal for charging a first one of said gate capacitors, a second control signal for charging a second one of said gate capacitors, a third control signal for turning on and off one of said lower switching devices, a fourth control signal for turning on and off the other lower switching device, a fifth control signal for discharging said first gate capacitor and a sixth control signal for discharging said second gate capacitor. 
     
     
       7. The lighting system of claim 6 wherein said upper and lower semiconductor switching devices are comprised of field-effect transistors. 
     
     
       8. The lighting system of claim 6 wherein said upper and lower semiconductor switching devices are comprised of insulated-gate transistors.